1. Technical Field
The present invention relates to the synchronization at multiframe level, on a determined traffic down channel, of a mobile terminal of a radiocommunications system.
2. Related Art
It finds applications, in particular, in cellular radiocommunications systems, for example professional radiocommunications systems or PMR (Professional Mobile Radiocommunication).
In such a system, a mobile terminal sometimes needs to change cell in the course of communication. In each cell of the cellular network, the base stations and the mobile terminals exchange traffic information (voice or data) over traffic paths exhibiting one and the same unique multiframe structure, a multiframe being a group of frames which repeats periodically.
A traffic down path serves for the transfer of information from the network to the mobile terminals. Conversely, a traffic uplink serves for the transfer of information from the mobile terminals to the network. In general, a traffic uplink is associated with a traffic down path, to allow duplex communications.
Within one and the same cell, all the information sent is synchronized at the multiframe level. However, in certain systems, the cells of the network are not synchronized with respect to one another.
This is why, so that the terminal changes cell while losing the least possible traffic (in send mode for a sending terminal or in receive mode for a receiving terminal), it must, preferably prior to this change, have recovered the synchronization at multiframe level of the destination traffic path (that is to say the traffic path over which the communication in progress is set up in the destination cell, and over to which it is getting ready to switch). For this purpose, it must acquire:                synchronization at frequency level;        synchronization at bit level (recovery of the bit time);        synchronization at frame level (recovery of the start of each frame); and        synchronization at multiframe level (index number of the frames in the multiframe).        
When a mobile terminal has recovered synchronization at multiframe level, it knows the structure of the logical channels of the traffic path and the corresponding frame formats: it knows where the traffic frames (voice or data) are and where the signalling frames are, their role, for whom they are intended, etc. The mobile terminal can then again participate in the communication in the destination cell.
The carrier frequency of the destination traffic path is communicated to the mobile terminal by the network. Synchronization at bit level and synchronization at frame level are conventionally recovered by monitoring a beacon path (MCCH channel, standing for “Main Control Channel”) sent continuously in the destination cell.
However, techniques based on the monitoring of the beacon path are not in general suitable for the recovery of synchronization at multiframe level, on account of the narrowness and of the small number of the monitoring windows at the disposal of the mobile terminal.
This is why mechanisms have already been proposed for searching for synchronization at multiframe level based on a signalling down channel in which information respectively associated with each frame of the traffic down path is despatched, and which allow the mobile terminals to recover the synchronization at multiframe level. Once the terminal is able, by means of this information, to identify any frame in the multiframe unequivocally, the synchronization at multiframe level is recovered.
A mechanism of this type is described, for example, in European Patent Application EP-A-1 094 615, within the context of a CDMA system. Each time slot is divided into two subfields one of which contains a synchronization code and the other of which contains useful signalling or traffic information (voice or data). The synchronization code is different in each time slot of the multiframe and is indicative of the start of the multiframe. Nevertheless, the synchronization codes must be coded on a number of bits which may become significant if the number of frames in the multiframe is high. This penalizes the bandwidth of the system, that is to say the throughput of useful information.